The present invention relates to a varifocal lens which is controllable in focal length and suitable for use in, for example, bar code readers.
Various types of varifocal lenses having a controllable focal length have been proposed in related art. For instance, JP-A-55-36857 (unexamined publication of Japanese patent application) discloses a structure comprising a functional liquid such as silicone oil enclosed in an airtight chamber defined between a pair of transparent elastic films faced to each other with a predetermined spacing provided therebetween. In this varifocal lens, pressure is applied to the operating liquid charged inside the chamber defined between the pair of transparent elastic films to deform the transparent elastic films into a convex shape. In this manner, for example, a convex lens having a bulged center portion can be obtained.
The morphology of such lens as described above changes depending on the rigidity of the transparent elastic films. In a case where the flexural rigidity is predominant in the transparent elastic films, the films undergo deformation to give a fourth order curved plane. In a case where tensile strength is predominant, the transparent elastic films deform to yield a spherical shape.
The varifocal lens must have a quick response from the viewpoint of making it suitable for various applications. Quick response varifocal lenses can be achieved by using a highly rigid material. However, because of the pressure applied to the operating liquid, the use of transparent elastic films made of a rigid material results in the formation of a lens having a fourth order curved surface. The formation of such a lens leads to the generation of aberrations which make it difficult to read, for example, bar codes.